www.ukbutterflies.co.uk

Here's a question I've always wondered about - theories greatfully received...
Why do male Fritillaries (* Note edit later) have only 4 walking legs, whilst females have 6? I know the males front legs have evolved into brushes for "tasting" food, but why did that only happen in male butterflies? Or are the brushes used to detect pheromones produced by females and hence the females don't need them? There must be a sex-related element to the evolutionary adaptation, so is it as simple as that?

Trev - as far as I am aware both female and male Frits have only 4 walking legs, like all nymphalids?

It is at least theoretically possible for a beneficial mutation to spread only amongst one sex if it arose under certain conditions on the determining sex chromosome. However, in butterflies sex determination is ZW (ZZ = male, ZW = female), so you would expect to see such a mutation in females rather than males.

... * As usual, I'm getting myself confused ... It isn't fritillaries I meant, but Duke of Burgundy (which used to be called a fritillary). But, I did get part of it right, the males DO only have 4 walking legs and the females 6... The plot thickens.

You need to read “The Butterflies of Sussex”

The theory is that the female needs 6 functional eggs to help her hold on to a Primula leaf when laying an egg on the underside of the leaf.

However, some Vanessids also do this, such as Small Tortoiseshell, which only have 4 fully-functional legs.

Cheers,

- Pete

I'm sorry Pete... I really must try to keep up (and that'll teach me for not yet buying the book which I meant to when it first came out). So, if some 4-footed females can reach around, is that because they have longer/more flexible abdomens compared to the stubby little "Mrs.Dobby"? I'm wondering if The Kipper or Padders have any other theories now. I'm still intrigued.

... and the 4 remaining "rear" legs would still be set as far back as they would be if there were 6 as it is the front legs which are adapted, so would it really make that much difference?.. I can potentially see that 2 more front legs may help to stop the butterfly from "tipping backwards" whilst trying to lay an egg around a leaf, but that seems really dubious to me. Maybe there is another theory which someone can come up with if we all put our heads together

I am by no means an expert at all but my book says that all male Lycaenidae use their back four legs to stand on and don't use their front two. It is clear that they stand on their back four, I have seen this for myself, but I have also seen male Silver-studded Blues hold onto females from the front with their front legs to try and keep them for themselves when there are other male suitors about but the female is not yet in-cop with them. In this single case at least it appears they are using their front pair of legs more like we use our arms!

Curious indeed.

Aha - I'm with you now!

Well, I have no idea of the function, but according to the most recent phylogeny of butterflies (https://www.sciencedirect.com/science/a ... via%3Dihub) the Duke is more closely related to the blues than to the Nymphalids. That would seem to me to suggest that the loss of the front legs in Riodinidae has not necessarily happened for the same reason, or at the same time, as it has in the true fritillaries.

Just to add a little spice to the recipe: the Libytheinae - now considered part of the Nymphalidae - also show sexual dimorphism in leg number. Females have what often looks like an ‘extra pair’:


(count carefully)



Guy

Hi Trev

"I can potentially see that 2 more front legs may help to stop the butterfly from "tipping backwards" whilst trying to lay an egg around a leaf, but that seems really dubious to me."

This is discussed in The Butterflies of Sussex (p.222): "Both sexes, of course, have six legs, but the front pair in the male is reduced to such an extent that these appendages are barely visible, and incapable of providing support. It took more than ten years of studying this species before it suddenly dawned on me why the Duchess retains a fully developed front set; this pair is essential in providing stability as she shuffles backwards to the edge of a Primula leaf and swings her body out and under, in order to position the eggs on its lower surface. By curving her abdomen into a wide arc the ova are deposited away from the very edge of the leaf, thereby reducing the risk of loss through incidental grazing by slugs and snails."

Furthermore, I believe that the male's front pair of legs have become vestigial, as, fully developed, they are surplus to requirements. Weight saving = increased speed; the primary factor in grabbing mating opportunities in this species.

BWs, Neil

P.S. You'll need to be quick; we've nearly sold out and there won't be a reprint.

OK Neil... You've cured my dubiousness with your very detailed (and typically thorough) explanation - I knew you would

... and the book should hit my doormat early next week

Best wishes.